Antivirus Quarantine Flaws Allow Privilege Escalation

Several popular antivirus products are affected by a type of vulnerability that allows an attacker to escalate privileges on a compromised system by abusing the quarantine feature, a researcher warned on Friday.

Once an attacker hacks into a system, they might need to somehow obtain higher privileges in order to access information that would allow them to move laterally within the network.

Florian Bogner, information security auditor at Austria-based Kapsch, claims to have discovered a new way to achieve this: abusing the quarantine feature of some antiviruses.

The attack method, dubbed by the researcher AVGater, relies on a combination of flaws and known techniques.

According to Bogner, an attack starts with a malicious DLL file being placed into quarantine by the antivirus software. The attacker then abuses the security application’s Windows process, which typically has SYSTEM permissions, to restore the file. However, the malicious DLL is not restored to its original location, but to a different folder from which a privileged process is launched – such as the Program Files or Windows folders – and where files cannot be written by a user with limited privileges.

Writing the restored file anywhere on the system is possible due to junctions, a type of file link supported by the NTFS file system. Junctions are file system representations that can be used to link directories.

Once the malicious DLL is placed in the targeted folder, the privileged Windows process associated with that folder will execute it instead of the legitimate file due to how the DLL search order works – Windows first looks for a DLL in the directory from which the app is loaded.

The vulnerability has been confirmed to affect products from Trend Micro, Emsisoft, Kaspersky Lab, Malwarebytes, Check Point (ZoneAlarm) and Ikarus, the expert said. Software from other vendors is impacted as well, but their names will only be disclosed after they release patches.

Bogner has published two separate blog posts detailing exploitation against Emsisoft and Malwarebytes products. In these examples, the attacker could have placed the malicious DLL in the directory associated with these security products so that the Emsisoft Protection Service and the Malwarebytes Service process, respectively, would load the malware instead of the legitimate library.

The researcher has not specified when other antivirus vendors were notified, but Emsisoft and Malwarebytes were informed in late 2016 and early 2017 and they released patches within a week.

Bogner pointed out that the AVGater vulnerability can only be exploited if the user whose account has been compromised can restore quarantined files. That is why he has advised organizations to ensure that regular users cannot complete such operations.

Eduard Kovacs is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.